On a budget, weigh it — Plaza de Toros de Las Ventas isn't cheap for what it is.
~€15 tour; corrida tickets vary widely
Architecturally it's a stunning Moorish-revival arena, and the guided tour or museum gives real insight into a contested tradition. The catch is the obvious one: an actual corrida involves the killing of bulls, which many visitors find ethically unacceptable — the building impresses, the spectacle divides hard.